U.S. Pat. No. 4,743,850 (Sepponen) discloses a process for spatial determination of the relaxation time T.sub.1p in a rotating frame of reference. In this process, the sample is first irradiated with a .pi./2 pulse in order to generate a transverse magnetization, and then with a spin lock pulse at a 90.degree. angle to the .pi./2 pulse, both of the pulses being applied in the absence of gradients. The pulse application is followed by volume selection by means of applied gradients, and lastly by signal evaluation. The problem with this process is that strong radio-frequency and gradient fields are needed for layer- or volume-selective excitation. These strong fields are undesirable for in vivo applications.
The object of the present invention is to make available processes in which strong radio-frequency and gradient fields are not needed for layer- or volume-selective excitation of a sample for in vivo investigations, and in which it is possible to acquire a reliable and versatile signal from the region of interest.